Devices may sense the presence (or absence) of particular molecules. For example, a miniature or hand-held spectrometer might be used to detect biological, chemical, and/or gas molecules. Such devices might be useful, for example, in the medical, pharmaceutical, and/or security fields. By way of example, a hand-held device might be provided to detect the presence of explosive materials at an airport.
In some sensing devices, light reflected from a sample of molecules is analyzed to determine whether or not a particular molecule is present. For example, the amount of light reflected at various wavelengths might be measured and compared to a known “signature” of values associated with that molecule. When the reflected light matches the signature, it can be determined that the sample includes that molecule.
In some sensing devices, a Fabry-Perot filter, or etalon, such as the one illustrated in FIG. 1 is used to analyze light reflected from a sample of molecules. The filter 100 includes a first partially reflecting mirror 110 and a second partially reflecting mirror 120 that define a resonant cavity C. Broadband light enters the filter 100, and some photons reflect off of the first mirror 110 while others pass through the mirror 110 and enter the cavity C. While in the cavity C, the photons bounce between the first and second mirrors 110, 120, and eventually some of the photons pass through the second mirror 120 and exit the filter 100.
As the photons bounce within the cavity C, interference occurs and an interference pattern is produced in light exiting the filter 100. As a result, light having a specific wavelength may exit the filter 100. Note that the interference occurring within the cavity C is associated with the distance d between the two mirrors 110, 120. Thus, the filter 100 may be “tuned” to output a particular wavelength of light by varying the distance d between the mirrors 110, 120 (e.g., by moving at least one of the mirrors 110, 120). By measuring light reflected from a sample using various distances d (i.e., at various wavelengths), and comparing the results with a known signature of values, a spectrometer associated with the Fabry-Perot filter 100 may determine whether or not a particular molecule is present in a sample.
One figure of merit associated with the Fabry-Perot filter's transmission bandwidth is “finesse,” which can be defined as FSR/FWHM where FSR represents the free spectral range of the Fabry-Perot filter 100 and FWHM is the width of the transmission peak at its 50% point.
The finesse of the Fabry-Perot filter 100 may be a function of the reflectivity (R) of the mirrors 110, 120. For an ideal etalon or interferometer, finesses may be associated with π(R)1/2/(1−R). Note that imperfections in the optical surfaces of the mirrors 110, 120 errors in parallelism between the mirrors 110, 120, and inclusions and imperfections in any material between the reflective surfaces of the mirrors 110, 120 may reduce the finesse of the Fabry-Perot filter 100.
To achieve proper performance for some applications, the filter 100 may need to have a relatively large finesse (e.g., to detect trace amounts of a substance). For example, to achieve a resolution of less that 5 centimeters (cm)−1 and a tuning range of more than 100 nanometers (nm) for a spectrometer, the finesse of the filter 100 may need to be relatively large (e.g., in excess of 100). Moreover, relatively small distances d may increase the need for high finesses and highly reflective mirrors 110, 120.
Because finesse is associated with π(R)1/2/(1−R), one way of increasing it's value to increase the reflectivity R of the mirrors 110, 120. For example, mirrors 110, 120 that are very reflective (e.g., 99%) may provide a relatively large finesse. In some cases, however, the mirrors 110, 120 may be formed of silicon, which has a relatively poor reflectance of approximately 25%. The mirrors 110, 120 could be coated with gold or silver to improve performance these materials might only improve the reflectance of the mirrors 110, 120 to approximately 98%—which might not be sufficient for some applications.